Rockwell Automation 1766-Lxxxx MicroLogix 1400 Programmable Controllers User Manual - Manual (Page 25)

Rockwell Automation Publication 1766-UM001H-EN-P - May 2014 11
Install Your Controller Chapter 2
• The master control relay must be able to inhibit all machine motion by
removing power to the machine I/O devices when the relay is
de-energized. It is recommended that the controller remain powered even
when the master control relay is de-energized.
• If you are using a DC power supply, interrupt the load side rather than the
AC line power. This avoids the additional delay of power supply turn-off.
The DC power supply should be powered directly from the fused
secondary of the transformer. Power to the DC input and output circuits
should be connected through a set of master control relay contacts.
Periodic Tests of Master Control Relay Circuit
Any part can fail, including the switches in a master control relay circuit. The
failure of one of these switches would most likely cause an open circuit, which
would be a safe power-off failure. However, if one of these switches shorts out, it
no longer provides any safety protection. These switches should be tested
periodically to assure they will stop machine motion when needed.
Power Considerations The following explains power considerations for the micro controllers.
Isolation Transformers
You may want to use an isolation transformer in the AC line to the controller.
This type of transformer provides isolation from your power distribution system
to reduce the electrical noise that enters the controller and is often used as a
step-down transformer to reduce line voltage. Any transformer used with the
controller must have a sufficient power rating for its load. The power rating is
expressed in volt-amperes (VA).
Power Supply Inrush
During power-up, the MicroLogix 1400 power supply allows a brief inrush
current to charge internal capacitors. Many power lines and control transformers
can supply inrush current for a brief time. If the power source cannot supply this
inrush current, the source voltage may sag momentarily.
The only effect of limited inrush current and voltage sag on the MicroLogix 1400
is that the power supply capacitors charge more slowly. However, the effect of a
voltage sag on other equipment should be considered. For example, a deep voltage
sag may reset a computer connected to the same power source. The following
considerations determine whether the power source must be required to supply
high inrush current: